1. Field of the Invention
The present invention relates to atrial synchronized ventricular pacemakers which can generate ventricular pacing events in response to sensed atrial events. More particularly, the invention relates to a pacer which modifies its operation to terminate pacemaker-mediated tachycardia.
2. Description of the Prior Art
Atrial synchronized ventricular inhibited pacing modes have been widely adopted for pacing therapy. The principal atrial synchronized operating mode is referred to as "DDD". This mode has a sensor augmented variant mode called "DDDR".
A DDD pacemaker includes an atrial sense amplifier to detect atrial depolarizations of the heart. If, after an appropriate delay, the ventricle of the heart fails to beat, the pacemaker supplies a ventricular pacing stimulus to the ventricle through an appropriate lead system. Pacemakers which perform this function have the capability of tracking the patient's natural sinus rhythm and preserving the hemodynamic contribution of the atrial contraction over a wide range of heart rates.
The principal problem associated with atrial tracking pacing modalities is referred to in the industry as "PMT" for Pacemaker-Mediated Tachycardia. In many patients, a ventricular stimulus may result in retrograde conduction from the ventricle to the atrium. If retrograde conduction reaches the atrium while the atrial tissue is refractory, the atrium does not depolarize, and no PMT occurs.
However, if the atrial tissue is not refractory, the retrograde conduction can stimulate a "beat" of the atrial tissue which may be detected by the pacemaker's atrial sense amplifier. Consequently, the pacemaker synchronizes to the retrograde "beat" and paces the ventricle again, after an AV delay interval. In this instance, the atrial sense amplifier is not detecting natural sinus depolarizations of the atria, but rather it is detecting atrial depolarizations caused by retrograde conduction. Consequently, the pacer is no longer following a natural atrial rhythm, but rather the pacing rate is determined by the retrograde conduction time of the patient and the AV delay interval of the pacer. This process results in ventricular pacing at an inappropriate rate which is at or below the upper ventricular rate limit of the pacer, and an inappropriate atrio-ventricular sequence.
A variety of events may initiate PMT, including: the occurrence of premature ventricular contractions (PVC's); pacer Wenkebach prolongation of AV interval; loss of atrial capture; atrial sensing of noise, and long programmed AV intervals.
Many approaches have been adopted to address the PMT problem including the techniques taught by:
U.S. Pat. No. 4,788,980 to Mann et.al., which teaches a retrograde sense period, invoked by the detection of a premature ventricular contraction (PVC). An atrial event detected in this window results in the generation of an atrial paced event if no intervening atrial or ventricular event is sensed. This patent teaches the suspension of the pacemaker's maximum tracking interval (MTI) after the detection of a PVC. This patent also teaches the shortening of the AV interval in response to the detection of a PVC.
U.S. Pat. No. 4,539,991 to Boute, teaches a pacemaker which monitors the atrial rate, then examines the stability of the VA conduction time to ascertain whether the pacer is participating in a pacemaker-mediated tachycardia at the ventricular rate limit.
U.S. Pat. No. 4,401,119 to Herpers, discloses a pacemaker which avoids PMT by extending the atrial refractory period upon the detection of a PVC.
U.S. Pat. No. 4,554,920 to Baker, et al., teaches a pacemaker which avoids PMT by extending the atrial Refractory period upon the occurrence of specific events, such as a PVC, noise reversion, or a programmed mode change.
U.S. Pat. No. 4,712,556 to Baker, teaches a pacemaker which inhibits a ventricular paced event after a predefined number of successive ventricular paced events have occurred at the ventricular rate limit.